1. Field of the Invention
The present invention generally relates to a rechargeable battery pack having a plurality of secondary cells, and in particular to a protection control technique against overcharging and over-discharging
2. Description of the Related Art
Recently, a rechargeable battery or a secondary battery has been widely used as a power supply for portable or hand-held wireless communication equipment in consideration of running costs.
In general, a battery pack having a secondary battery such as lithium-ion battery therein is provided with a protection circuit for protecting the secondary battery from overcharging, over-discharging and overcurrent.
An example of such a protection circuit has been disclosed in Japanese Patent Laid-open No. 8-116627. This conventional protection circuit is provided with a first controller and a second controller. The first controller performs the on/off control of a switching circuit according to the output voltage of the secondary battery to adjust the charging current of the secondary battery. The second controller controls a cutoff circuit or a circuit breaker according to the charging voltage. More specifically, when the second controller judges that a fault condition occurs in the first controller under overcharging conditions, the second controller activates the circuit breaker so that the charging current is cut off. The circuit breaker is designed to automatically break and manually make a circuit. In this conventional protection circuit, a fuse circuit is used.
According to the above conventional protection circuit, however, the first and second controllers monitor the output voltage of the secondary battery to determine whether overcharging or over-discharging occurs. Therefore, if at least one of the first and second controllers is in fault conditions, resulting in incorrect detection of the battery output voltage, then there is a possibility that the secondary battery is subjected to overcharging or over-discharging.
Further, when detecting a charging voltage higher than a threshold, the first controller controls the switching circuit to turn it off. But, in case of the switching circuit being out of order, there is a possibility that the charging current is not cut off, resulting in the occurrence of overcharging or over-discharging.